This invention relates to a method and to an apparatus for controlling fuel of a gas turbine and, more particularly, relates to a method and to an apparatus for controlling fuel at the time of load rejection or FCB (first cut back).
In machinery combining power and load or in a combination plant of a generator and a prime mover serving as power source, for example, load rejection is when the load of the generator is reduced abruptly down to zero or a certain fixed load while keeping a ordinary or normal number of revolutions. In the event of such load rejection, it becomes necessary to control the prime mover to supply thereto less energy.
For example, in rotary machine such as a steam turbine, with load rejection, generated energy is converted to rotational energy to bring about an increase in the number of revolutions if the steam supply is not decreased. However, the rotary machine has an upper limit with the number of revolutions which is decided by the allowable stress of its material so that it is necessary to control the steam supply valve to be closed or opened at a proper opening for the purpose of preventing the number of revolutions from exceeding the upper limit. On the other hand, as the load rejection takes place in the gas turbine, it becomes necessary to reduce the fuel supplied to the gas turbine for the same reason as described above. It is noted here that the gas turbine is constructed in general by a compressor, a combustor and a turbine. Fuel is injected into the combustor to be burned with air compressed to high pressure by the compressor. The combustion gas is then expanded in the turbine to generate or perform work. Accordingly, since it is necessary to produce compressed air by the compressor at all times in order to maintain a constant number of revolutions of the gas turbine, work must be performed by the turbine by an amount corresponding to compression work even in the state of no load. Thus, it becomes necessary to inject a prescribed quantity of fuel even if the turbine is operated with no load. Further, in the combustor of the gas turbine, combustion cannot take place as the flow rate of fuel to be supplied to the combustor falls below a certain limit, resulting in a phenomenon known as flameout. Such flameout results in the air containing combustible matter and flowing downstream to be in an extremely dangerous state. For this reason, it is one of the necessary functions of the fuel flow rate control valve controlling apparatus to rapidly reduce the time required from the opening of the fuel flow rate control valve during normal operation to the opening of the fuel flow rate control value at which the fuel flow rate corresponds to no load operation without causing the flameout phenomenon and incurring a excessive number of revolutions at the time of load rejection.
FIG. 3 shows an example by a block diagram of a conventional fuel flow control system. In this system, an output W detected by an output detector 6 attached and belonging to a load 9 is compared with a set output W* as a desired or target value by a comparator 1 and is then converted into a desired number of revolutions N* by an arithmetic unit 2 in accordance with an output deviation .DELTA.W thus obtained. In the gas turbine, this conversion function is usually set at 104% when a rated output is the desired value and at 100% for no load. The desired number of revolutions N* is increased at a predetermined fixed rate when the output deviation .DELTA.W is positive, and N* is decreased at a predetermined fixed rate when .DELTA.W is negative.
However, when no power is required for the load 9, it is set at 100%.
A number of revolutions N detected by a revolution detector 7 belonging to a gas turbine 8 is compared with the desired number of revolutions N* by a comparator 3, and an output .DELTA.N thus obtained is converted into the desired opening .theta..sub.0 of fuel flow rate control valve by an arithmetic unit 4.
An arithmetic unit 5 calculates the minimum value of the desired opening of a fuel flow rate control valve 12 to set the opening of the fuel flow rate control valve within a range that can prevent a combustor 14 from causing a flameout. The flameout of the combustor 14 happens depending on the characteristics of the combustor; it takes place when the fuel air ratio becomes equal to or less than a certain predetermined value Since the flow rate of the air discharged from a compressor 13 to be supplied to the combustor 14 varies in accordance with the atmospheric temperature and the number of revolutions, the arithmetic unit 5 corrects for the minimum value of the desired opening of the fuel control valve 12 in response to the atmospheric temperature T detected by an atmospheric temperature detector 11 and number of revolutions N.
A pressure control valve 17 keeps constant an inlet pressure P.sub.2 of the fuel flow rate control valve 12 detected by a pressure detector 18. Therefore, the fuel flow rate which decides the output (power) of the gas turbine is determined uniquely by the opening of the fuel flow rate control valve 12.
In accordance with the change of .theta..sub.1 which is the output of the arithmetic unit 5, the opening of the fuel flow rate control valve 12 is varied to change of the fuel flow rate and, thereby makes the output W approach the set output W*. In this way, a prescribed feedback control is performed.
Here, FIG. 4 shows the behavior of the above conventional control system in the case that load is rejected or the set output W* for the load 9 becomes zero from the rated value in substantially an instant.
Description will be given below of FIG. 4. It is assumed that the load rejection takes place at a point of time T.sub.1. The set output W* changes from point A.sub.1 to point A.sub.3 via point A.sub.2. Assuming that A and B are constants, the calculation is done in the arithmetic unit 4 based on the following formula; EQU .theta..sub.0 =A.times.(N*-N)+B
Values for B correspond to lines g and h, for example, conditions without load.
A value for the desired minimum valve opening corresponding to line e is set in the arithmetic unit 5. Therefore, the desired fuel flow rate control valve opening .theta..sub.1 changes from point B.sub.1 to point B.sub.3 via point B.sub.2, thereafter, passes through point B.sub.4 and asymptotically approaches line g. The opening of the fuel flow rate control valve 12 changes from point C.sub.1 to point C.sub.3 via point P, and thereafter, gets to line h in accordance with the change of the desired opening. The output (power) W of the gas turbine changes along a line equivalent to line c. During the period t.sub.1 in which the output of the gas turbine is not less than zero after the load is rejected at point T.sub.1, the number of revolutions N rises temporarily form point D.sub.1 to point D.sub.2, and however, it tends to drop as the opening of the fuel flow rate control valve 12 settles down to line d and, thereafter, settled down to line h.
In this way, the output W responds to the change of the set output W*.
According to this controlling method, however, the minimum opening of the valve becomes less than the minimum set value for the desired or target opening value, resulting in that fuel is supplied to the combustor only at the rate below the desired fuel flow rate. To cope with this, the limit can be fixed beforehand, which must be fixed to line f in actuality, to line e obtained by upwardly correcting by an amount of difference .DELTA.B corresponding to a difference .DELTA.C between lines d and b. However, this measure gives rise to a problem that the rise of the number of revolutions is further promoted at the time of load rejection so that it cannot be regarded as a solution.